1. Field of the Invention
The present invention relates to a device for the controlling electrical power supply to an oxygen pump forming part of a linear oxygen probe apparatus and, more particularly, to such a device for measuring the oxygen concentration in exhaust gases from an internal combustion engine.
2. Description of the Related Art
It is known that an important parameter in the operation of an internal combustion engine is the air coefficient lambda (.lambda.) which represents the air:fuel ratio in the mixture supplied to the engine divided by the ratio corresponding to a stoichiometric mixture. The importance of this parameter comes from its effect on the engine power, on the engine's specific consumption and, above all, on the composition of the exhaust gases from the engine, particularly noxious gases. Antipollution laws of ever greater severity oblige engineers to provide for regulation of this coefficient .lambda., making it possible to obtain as complete a combustion of the mixture as possible.
To this end, the composition of the latter is measured indirectly by detection of the amount of oxygen present in the exhaust gases. Given that these gases contain oxygen even when the mixture is "rich" (.lambda.&lt;1), the measurement of this amount of oxygen enables the composition of the mixture supplied to the engine to be measured.
For this purpose, a sensor known as an "oxygen probe" or "lambda probe" is used, the probe being placed in the flow of exhaust gases from the engine. Conventionally, such a probe consists of a mass of zirconium or titanium oxide placed between two electrodes, with one of the electrodes being exposed to the exhaust gases through a protective layer of porous ceramic while the other electrode is in contact with the open air. A voltage is observed between the electrodes which is a function of the difference in the oxygen ion concentration between the two electrodes. In particular, a jump in voltage is observed at around .lambda.=1, due to the particular properties of the material used, whether zirconium oxide or titanium oxide.
As a result of this jump, such a probe supplies a signal which in practice takes on only two states, representative of a mixture in which either .lambda.&lt;1 or .lambda.&gt;1. It is, therefore, impossible to measure the real departure of the mixture from its stoichiometric composition.
To overcome this drawback, an oxygen probe described as "linear" has been designed which is capable of supplying a signal which is substantially linearly proportional to the coefficient .lambda. both above and below the stoichiometric composition.
Such a linear oxygen probe normally comprises, as represented in a portion of FIG. 1 of the appended drawing, a measurement cell 1 and an oxygen pump 2 separated by a measurement cavity 3, the measurement cell 1 being itself bounded on its other face by a reference cavity 4. The measurement cell 1 consists of a standard oxygen probe comprising a body 5 made of zirconium or titanium oxide, for example, and two electrodes 6 and 7 placed against this body, in the measurement cavity 3 and in the reference cavity 4, respectively. The measurement cell delivers a voltage V.sub.s across these electrodes which depends on the amount of oxygen present in the measurement cavity. The measurement cavity 3 is in communication with the exhaust gases from the engine, as is a volume 8 which surrounds an electrode 9 of the oxygen pump whose other electrode 10 lies in the measurement cavity 3. The oxygen pump 2 comprises, like the measurement cell 1, a mass of zirconium or titanium oxide placed between the electrodes 9 and 10.
Passing a current I.sub.p through the oxygen pump 2 causes oxygen to arrive in the measurement cavity 3 or causes it to be extracted from this cavity 3, depending on the direction of the current. By bringing this current and its direction under the control of the voltage V.sub.s delivered by the measurement cell 1, a closed loop regulation of the oxygen concentration in the measurement cavity 3 is ensured by a supply of oxygen to, or a withdrawal of oxygen from, this cavity depending on the oxygen concentration in the exhaust gases entering this cavity. The current I.sub.p is then a substantially linear function of this oxygen concentration and, thus, of the coefficient .lambda. representative of the composition of the air-fuel mixture. As an example of a known probe which operates according to this principle, the probe sold under the name NTK UEGO by the Japanese company NGK SparkPlug Co. Ltd may be cited.
The problem is then to bring the current I.sub.p supplied to the pump under the control of the voltage V.sub.s delivered by the measurement cell. For this purpose, use is conventionally made of a complex PID-type analog circuit consisting of many analog amplifiers and precision components. Such an analog circuit is subject to drifting. Its complexity and the presence of precision components make the cost prohibitive. Moreover, since the functioning of the feedback control is fixed by the structure of the circuit, this functioning cannot be modified by using various strategies programmed in advance, as is the case with digital systems controlled by a microprocessor, for example.